1. Field of the Invention
The present invention generally relates to magnetic bearing systems, and more particularly to a system to dampen mechanical oscillation in the shaft supported by the bearing, at critical shaft speeds.
2. Description of the Prior Art
As will be appreciated by those skilled in the art, a flexible rotating shaft, such as a typical motor or generator shaft, has a tendency to vibrate due to unavoidable unbalance forces. The vibration causes a oscillation of the shaft about its longitudinal axis. For a given unbalanced force, the frequency of the oscillation is a function of rotational velocity. The resonant frequency of the shaft is a function of shaft parameters such as stiffness and shaft length between the bearings. Thus, as the shaft speed changes the frequency of shaft oscillation changes; as the speed increases the frequency increases.
As will also be understood by those skilled in the art, shaft bearings constrain the shaft and tend to dampen these oscillations except at certain critical shaft speeds at which the shaft oscillates at the resonant frequency of the supported shaft. In this situation an oscillation node occurs at the bearings and the bearings exert little or no dampening force on the shaft since there is no shaft displacement at the bearing locations in this situation.
There are a number of magnetic bearing systems in the prior art in which the energization of the bearing is controlled to dampen shaft oscillation. These prior art magnetic bearing systems do not exert a damping force on the shaft at those critical shaft speeds at which an oscillation node and the bearings coincide.